This invention relates to appliances for cooking foods, and more particularly, to a control and control methodology for the appliance and heating units installed in the appliance to control operation of the heating units at turn-on and at other stages in a cooking cycle.
Electrical heating units, as is well-known in the art, comprise an electrical heating element such as a coil heating element, or a ribbon heating element. Heating units are available in different wattages and it is common for a cook top to have heating units of more than one wattage rating. The heating element is mounted on, or secured to, a cake of insulation material which, in turn, is fitted in a pan. The ends of the heating element connect through a thermal switch to an electrical circuit by which current is supplied to the heating element. The unit is installed beneath a heating surface upon which utensils are placed. Heat generated by the heating element is transferred to the heating surface by radiation, and from the heating surface to the utensil by conduction. The thermal switch includes a temperature sensing element which, if it senses the heating unit temperature exceeding a preset temperature, opens the switch and cuts-off current flow to the heating element. Besides this open loop arrangement employing the thermal switch, other heating unit controls employing temperature sensing have been tried. In coassigned U.S. Pat. No. 5,397,873, an electrical heating apparatus is described employing a temperature sensor installed in direct contact with a cooking surface. The apparatus is responsive to temperature changes at the cooking area to facilitate cooking food. However, heretofore, cooking appliances have not employed a closed loop temperature control system that can maintain the cooking temperature of food within a narrow range of temperatures about a user selected cooking temperature.
There are a number of problems with existing heating units. For example, it is now desirable that when current is first applied to the heating element that the heating element rapidly warm to a temperature at which the element starts to glow. The time for this to happen is approximately 3-5 seconds. While rapid heating is a desirable product feature, the current methods by which this is accomplished also shortens the life of the heating element.
In addition to rapid heating, another useful feature is the ability of the heating unit to simmer foods. The current test for simmering is to place a utensil with chocolate or a sauce on a heating unit and set the unit temperature to predetermined simmering temperature. It is a problem with current heating units that regardless of the temperature control scheme employed, the chocolate or sauce usually scorches. Better control of simmering so there is no scorching is therefore a desirable feature.
As noted, current heating units employ a temperature responsive limit switch which acts to cut-off power to a heating unit when a predetermined temperature is exceeded. The limit switch assembly is expensive, representing approximately 20-30% of the total cost of the heating unit. The limit switch assembly also is a primary source of heating unit failure. Elimination of the switch would not only be a substantial cost savings, but would also impact the service life of a heating unit; provided, that proper temperature control of the heating unit is still maintained.